The present invention relates to a switch-type voltage regulator with reduction of occupied space for internal soft-start functions.
As is known, for switch-type voltage regulators, it is necessary to provide integrated circuit elements and external circuit elements which perform the so-called soft-start function, to prevent overloading of some components and causing dangerous output overvoltages during switch-on.
For greater clarity, reference is made to FIG. 1, which shows by way of an example a switch-type regulator 1 of a Buck-type circuit configuration.
In detail, the regulator 1 comprises an integrated circuit 100, which has an input terminal 2 receiving an input voltage VIN, an output terminal 3 connected to a load 4 and supplying an output voltage VOUT smaller than the input voltage VIN, a compensation terminal 25, and a soft-start terminal 26, supplying a soft-start voltage VSS. A compensation resistor 27 and a compensation capacitor 28, connected in series to each other, are coupled between the compensation terminal 25 and ground, whereas a soft-start capacitor 29 is connected between the soft-start terminal 26 and ground.
A DC-DC converter 5 is connected between the input terminal 2 and the output terminal 3 of the integrated circuit 100, and comprises a switch 6 (typically formed by a power transistor, opening and closing whereof is controlled by a driving stage 7), a diode 8, an inductor 10, and a filter capacitor 11.
The regulator 1 also comprises a voltage divider 15, connected between the output terminal 3 and ground, including a first resistor 16 and a second resistor 17, connected to each other at a feedback node 18 supplying a feedback voltage VFB, proportional to output voltage VOUT.
An error amplifier 20 supplies to an intermediate node 21 an error voltage VE which is function of the difference between the feedback voltage VFB and a constant reference voltage VREF, supplied by a voltage source 22.
The error voltage VE and a comparison voltage VC, supplied by an oscillator 32 and having a saw-tooth plot with pre-determined frequency, are supplied to the input of a differential-type comparator (PWMxe2x80x94Pulse Width Modulator comparator 30), which in turn generates at the output a driving signal VP, which is supplied to the input of the driving stage 7 of switch 6.
The intermediate node 21 is also directly connected to the compensation terminal 25 and to a soft-start circuit 35, comprising a current source 36, supplying a constant current ISS, and an enable transistor 37, for example a bipolar transistor of the PNP type. In particular, an output terminal of the current source 36 is connected to the soft-start terminal 26 and to a base terminal of the enable transistor 37, which also has an emitter terminal connected to the intermediate node 21, and a collector terminal connected to ground.
When the regulator 1 is switched on, all capacitors 11, 28, 29 are discharged, and the feedback voltage VFB is zero. The error amplifier 20 is completely unbalanced, but since the enable transistor 37 is on (in fact it has a base-emitter drop greater than its switching-on voltage), the error voltage VE of the intermediate node 21 is equal to the soft-start voltage VSS at the soft-start terminal 26, plus the base-emitter voltage of the enable transistor 37. Since the current source 36 charges the soft-start capacitor 29 with the constant current ISS, the soft-start voltage VSS and the error voltage VE increase linearly, until a predetermined threshold is reached, beyond which the enable transistor 37 turns off, and the soft-start circuit 35 switches off.
This prevents overvoltages at the output; in fact, in the absence of the soft-start circuit 35, the error voltage VE would immediately reach its maximum value, thus giving rise to the maximum possible duty-cycle, and inducing the power switch 6 to support the maximum current, with a risk of an output overvoltage. As an example, FIG. 2a shows a comparison between the plot of the error voltage VE in a regulator provided with a soft-start circuit (broken line), and the plot of the error voltage VE in a regulator without a soft-start circuit (dot-and-dash line); on the other hand, the continuous line represents the comparison voltage VC. FIGS. 2b and 2c show the plot of the driving voltage VP respectively in the case of absence of the soft-start circuit and in presence of the soft-start circuit.
However, the known switch-type regulators have a drawback caused by the need to use components external to the integrated circuit 100 (i.e., the soft-start capacitor 29), specifically in order to provide the soft-start function. This is particularly disadvantageous, since the presence of the relevant pin (soft-start terminal 26) and of the soft-start capacitor 29 involves a significant bulk and an increase in costs, which is particularly detrimental in portable electronic devices, and in particular in power supply for portable PCs.
The disclosed embodiment of the present invention provides a switch-type regulator with a soft-start function, free from the described disadvantages, and in particular allowing a reduction in the number of external components and pins.
According to the present invention, a switch-type voltage regulator is provided that includes an output terminal supplying an output voltage, a compensation network, and an error amplification circuit, the error amplification circuit having a first input receiving a reference voltage, a second input receiving a feedback voltage correlated to the output voltage, and an output connected to a compensation terminal of the compensation network, the error amplifier circuit supplying an error voltage correlated to the difference between the reference voltage and the feedback voltage, and further comprising a soft start circuit incorporated in the compensation network that is connected to the compensation terminal.
In accordance with another aspect of the invention, a switch-type regulator is provided that includes a converter circuit having a first input coupled to an input node, a second input, and an output coupled to an output node; a comparison voltage source having an output; a differential-type comparator having a first input coupled to the output of the comparison voltage source, a second input, and an output coupled to the second input of the converter circuit; and an error amplifier circuit having an input coupled to the output node to receive a feedback voltage, and an output coupled to the second input of the differential-type comparator, the error amplifier circuit comprising a comparator circuit having a first input coupled to the error amplification circuit input, a second input coupled to a first reference voltage, and an output coupled to the error amplification circuit output, and further including a differential amplifier having a first input coupled to the error amplifier circuit input, a second input coupled to a second reference voltage, and an output coupled to the error amplifier circuit output, the error amplifier circuit further coupled to a compensation circuit input terminal that in turn is coupled to the input node and to the output node, the error amplification circuit configured to supply an error voltage at its output that is correlated to the difference between the first reference voltage and the feedback voltage.